The present invention relates to nonlinear optical materials.
Nonlinear optical materials have actively been investigated as the important technology for optoelectronics in such fields as wavelength converter of laser beam, optical modulator, optical switch and optical computer. As the nonlinear optical material are well known lithium niobate, potassium dibydrogen phosphate and the like. Unlike these inorganic materials, there have been obtained no crystals of organic materials having a macroscopic second order hyperpolarizability large enough to allow for practical use, although they possess superior characteristics such as a nonclear optical constant 100-1000 times as high, response at a higher speed and stronger resistance to optical damages. In many case, even when the constructing molecule itself has a large secondary molecular susceptibility, the macroscopic second order hyperpolarizability will be zero in crystalline state due to its inversion symmetry with a result that there will be no optical secondary harmonic generation (SHG). As stated above, it is difficult to produce single crystal with no center of symmetry by the use of a low-molecular organic substance alone. Therefore, there have been proposed methods in which such organic molecules are dispersed in a matrix of a macromolecular compound and the host macromolecules are oriented by the aid of external field such as electric field simultaneously allowing for unsymmetric orientation of the guest organic molecules. For example, SHG is observed by blending 4-dimethylamino-4'-nitrostilben (DANS) in a nematic liquid crystalline macromolecules and then applying electric field to the bland to cause orientation [G. R. Meredith, Macromolecules, 15(5), 1385(1982)]. In this method, however, as the DANS can be blended in a concentration as low as 2% at highest, and orientating force of the host macromolecular liquid crystals is not satisfactorily high, there is produced an unsatisfactory nonlinear optical constant several times as high as that of urea. Japanese Patent Laid-open No. 238538/1987 (JP62-238528A) also describes an observation of an SHG of 0.53 .mu.m by incidence of Nd.sup.3+ /YAG laser beam upon a film prepared from a composition of DANS dispersed in thermotropic macromolecular liquid crystals of main chain type. Also in EP No. 244288 are described synthesis of a liquid crystalline polymer of side-chain type in which a mesogen and a unit exerting nonlinear optical response have been linked to the side chain of a polymer such as polyacrylate, polymethacrylate or polysiloxane and orientation of a film from the polymer by applying an electric field followed by incidence of Nd.sup.3+ /YAG laser beam by which an SHG of 0.53 .mu.m is observed. The methods described in these literatures are commonly characterized by the use of a matrix polymer that forms nematic liquid crystals and consequently have a critical disadvantage that a material with a nonlinear optical constant sufficiently high for practical use cannot be produced. Another disadvantage of the method using nematic liquid crystalline polymer is that use of nematic liquid crystalline polymer produces no benefit in the phase matching which is technologically important for practical use, and utilization of conventional birefrigence or optical waveguide is necessary.
On the other hand, use of polyethylene oxide which does not form liquid crystals as a matrix polymer has also been proposed [Miyata et al., Polymer Preprints, Japan Vol. 36, No.8, 2523(1987)]. An SHG 90 times as high as that of urea has been obtained by Miyata et al. by dispersing p-nitroaniline in polyethylene oxide and crystallizing the polyethylene oxide under electric field. As described by Miyata et al., however, it is a problem that intensity of the SHG begins to decrease in about 10 hours eventually to 10% of that of MNA (2-methyl-4-nitroaniline).